Then I've wanted to enabled/disable some according to specific conditions.
a) first, I've noted that autocomplete offers me old names - before renaming them
to compile had to correct to new action name
b) second, only part of the actions are listed (missing ones from step 3 in above list)

Not at all, Qt Creator doesn't impose such a restriction. The Qt version used to build Qt Creator has nothing to do with the one you use for development e.g. you can use the same for desktop/Android/iOS/embedded linux development. Except that if you want to use Qt 5.2 and upper you have to use Qt Creator 3 or above.

I remember having a similar problem, but I haven't tried with the latest version.